1. Field of the Invention
The field of the invention is that of enlarging optical lenses and, more precisely, binocular optical systems designed to be inserted into the night-time channel of an observation system, between a light intensifier tube and an observer's eyes.
The invention can be applied especially to observation instruments for armored vehicles and, notably, periscopes for tanks.
However, the optical system of the invention can naturally be applied whenever there is an optical context similar to the one described in detail further below.
In the particular case of armored vehicles, for daytime vision, the driver generally has at least one standard optical periscope known as a daytime channel, comprising an input reflector receiving the light rays, and an output reflector restoring the light rays to him. Depending on the type of periscope, the path of the light rays between the two reflectors may be direct or may undergo one or more reflections.
The daytime channel provides for optimum vision as long as there is sufficient lighting. However, it does not provide for night-time driving. For, it is not possible, at least in combat position, to use lighting means integrated into the vehicle. It is therefore necessary to use an electronic night-time channel that restores a visible image of the outside scene to the pilot and includes, for example, an objective, a light intensifier tube and a binocular device.
2. Description of the Prior Art
However, the known periscopes have dimensions that are too great for them to be adapted to existing day/night equipment. Apart from their major drawback of giving the pilot an image that is less clear than the one given by daytime periscopes, they use the place available between the pilot's eye and the armor, and thus prevent the interposition of a daytime channel.
Since a periscope needs to be easily replaceable, for example in the event of deterioration or malfunctioning or else quite simply in order to carry out maintainance adjustments, it generally takes the form of a compact unit including all its constituent elements (optical ray deflection reflector, mechanical frame for the night-time channel, objectives, light intensifier tube, daytime channel reflectors, etc.).
A variety of mechanisms may be used in order to provide a compact periscope having the smallest possible dimensions.
One mechanism that can be used to change from vision by daytime channel to vision by night-time channel is described, for example, in the French patent application No. 90 07838 relating to a "hinged mechanical frame, notably for periscopes, and periscope mounted on such a frame". The change from one viewing channel to another is done in such a way that the minimum amount of space is needed, by means of a compact mechanism for the control of the tilting of a reflector that directs the light rays selectively towards one channel or another.
For the same purpose, it has also proven worthwhile to consider reducing the size of the binocular devices with which the periscopes are fitted out. For, in a periscope, the binocular device is placed facing the user and thus occupies a relatively large amount of space. The space factor constraint may thus make it necessary for the optical system to include, for example, an elbowed part that enables the light rays emerging vertically from the intensifier tube to be deflected horizontally towards the user's eyes.
Apart from the space factor constraint, another factor governing the designing of a binocular device for a periscope is the need for compatibility with the other elements of the optical chain. Thus, for reasons of modularity and economy, it should be possible for the binocular devices to be matched with the dimensions of existing intensifier tubes, and notably with the dimensions of the periscopes fitted into tanks of the Leclerc, AMX 30 or T (trade names) types.
The designing of a optical system for the night-time channel of a periscope therefore requires a choice of an intensifier tube/binocular device pair.
There also exist known binocular devices for night-time channel periscopes coupled with 20/30 mm intensifier tubes, for example the XX 1380 intensifier tube by PHILIPS (registered name).
However, the field of vision provided by these binocular devices is not wide enough to give satisfactory comfort in the driving of an armored vehicle, and in particular the driving of a tank.
There also exist known formulae for binocular devices adapted to the widely used 25/25 mm intensifier tubes of the MX 9644/UV type, where the coupling between the intensifier tube and the binocular device is done by means of a 25/46 mm enlarging fiber.
The main drawback of this type of binocular device is that the enlargement ratio needed to bring the image to a sufficient diameter is 1.6. This leads to a reduction in the aperture of the fiber at output, and this reduction is incompatible with a wide aperture of the binocular device. This means a darkening of the central part of the field for a nominal position of the observer's eyes, and a loss in contrast due to the fiber. Furthermore, the result of this approach is too cumbersome for most day/night applications.
A third type of binocular system, adapted to periscopes, is applied in 25/40 mm tubes of the TH 9311 type by THOMSON (registered name). These tubes are well adapted to the criteria of space factor and image quality, but have the drawback of being very costly.
Furthermore, there is a known French patent document by ROGERS relating to optical systems for night vision devices, wherein the image is formed on a phosphorus screen, typically an intensifier tube, and is amplified by lenses. The optical systems described are of a type comprising three groups of lenses, all of which are convergent. The first group, which is called a forward group, is characterized by a focal length ranging from 3F to 12F, where F is the focal length of the optical system concerned, the second group, called a median group, has a focal length ranging from 1.6F to 2F and the third group, called a rear group, has a focal length ranging from ranging from 1F to 1.75F. The third group is constituted by a frontwardly convex positive meniscus, the rear face of which may have an infinite radius, i.e. it may be plane.
However, these optical systems cannot be used to obtain a sufficiently small aperture number, so much so that the observer must position his eyes in a reduced zone of space.
Furthermore, it is necessary to devise a radically novel optical formula because the range of efficiency of existing formulae cannot be extended indefinitely by simple quantitative modification. Indeed, for a given optical system, the operating limit is reached as soon as a limit size of lens is reached.